This invention relates to cleaning devices for mist eliminators in a wet scrubbing device. In one aspect it relates to a mist eliminator cleaning device for continual wash of the mist eliminator during operation. In another aspect the invention relates to a mist eliminator cleaning device for washing both upstream and downstream sides of the mist eliminator simultaneously.
In the removal of air polluting materials from dirty gas streams and particularly those which are removed in wet scrubbing devices by chemical reaction, these wet scrubbing devices include mist eliminators therein to prevent mist from being entrained in the gas stream as it leaves the scrubbing unit. These mists generally contain the reacting materials and in many instances the carryover of these liquid solutions in the form of a mist is often very great. For example, in the removal of oxides of sulfur, the scrubbing solutions usually contain calcium in the form of lime or limestone wherein the scrubbing solution including the calcium contacts the oxides of sulfur in the gas stream as the gas stream moves upward through a scrubbing device. Thus, mist is formed containing the oxides of sulfur, the calcium reacting solution, as well as stable products resulting from the reaction of the aforementioned reactive ingredients. The mist is generally trapped by mist eliminators while the chemical reaction is still proceeding. As a result of this contact with the mist eliminator, oxides of sulfur are absorbed in a relatively low pH entrained liquid and precipitation of solids occurs on the demister surfaces. This precipitation can occur rapidly and thereby causes blockage of the gas passage within the demisting elements.
Washing of the mist eliminators is necessary in order to prevent this blockage and is normally accomplished by use of a spray washer or wash lance, similar to a steam generator soot blower. In many installations it has been found that these mist eliminators must be cleaned at least once every 24 hours in order to maintain their effectiveness and this cleaning is generally done with the unit shutdown. A problem of washing is compounded in most mist eliminators by the fact that both upstream and downstream sides of the mist eliminators must be washed.
The most common types of mist eliminator means utilized in wet scrubbers include those having layers of screening, mesh, chain, spheres, or other types of packing which provide high surface area for capturing of entrained mist or droplets and another type, which is preferred in removal of oxides of sulfur, is one utilizing staggered members or closely spaced "zig-zag" shaped plates which traverse a gas passageway to provide an impingement surface area with a multiplicity of narrow, tortuous passageways. This type of mist eliminator arrangement has frequently been referred to as "chevron" packing because of this angular, zig-zag configuration. In either type of mist eliminator packing, cleaning of the mist eliminator is a problem and this cleaning problem is compounded by the fact that both upstream and downstream sides of the eliminator must be cleaned. Furthermore, if this cleaning is done while the scrubber is on line, the entrainment of liquid on the downstream side must be kept at a minimum. Also, when cleaning the mist eliminator while the scrubber is onstream, it is important to maintain the proper PH of the lime or limestone based scrubbing solution with the balance of water in the mist cleaning solution. Thus, only a small quantity of mist eliminator cleaning solution may be utilized.